dm: Add a README for of-platdata
Add documentation on how this works, including the benefits and drawbacks. Signed-off-by: Simon Glass <sjg@chromium.org>
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Simon Glass
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doc/driver-model/of-plat.txt
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doc/driver-model/of-plat.txt
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Driver Model Compiled-in Device Tree / Platform Data
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====================================================
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Introduction
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------------
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Device tree is the standard configuration method in U-Boot. It is used to
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define what devices are in the system and provide configuration information
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to these devices.
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The overhead of adding device tree access to U-Boot is fairly modest,
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approximately 3KB on Thumb 2 (plus the size of the DT itself). This means
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that in most cases it is best to use device tree for configuration.
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However there are some very constrained environments where U-Boot needs to
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work. These include SPL with severe memory limitations. For example, some
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SoCs require a 16KB SPL image which must include a full MMC stack. In this
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case the overhead of device tree access may be too great.
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It is possible to create platform data manually by defining C structures
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for it, and referencing that data in a U_BOOT_DEVICE() declaration. This
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bypasses the use of device tree completely, but is an available option for
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SPL.
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As an alternative, a new 'of-platdata' feature is provided. This converts
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device tree contents into C code which can be compiled into the SPL binary.
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This saves the 3KB of code overhead and perhaps a few hundred more bytes due
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to more efficient storage of the data.
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Caveats
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-------
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There are many problems with this features. It should only be used when
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stricly necessary. Notable problems include:
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- Device tree does not describe data types but the C code must define a
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type for each property. Thesee are guessed using heuristics which
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are wrong in several fairly common cases. For example an 8-byte value
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is considered to be a 2-item integer array, and is byte-swapped. A
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boolean value that is not present means 'false', but cannot be
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included in the structures since there is generally no mention of it
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in the device tree file.
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- Naming of nodes and properties is automatic. This means that they follow
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the naming in the device tree, which may result in C identifiers that
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look a bit strange
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- It is not possible to find a value given a property name. Code must use
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the associated C member variable directly in the code. This makes
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the code less robust in the face of device-tree changes. It also
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makes it very unlikely that your driver code will be useful for more
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than one SoC. Even if the code is common, each SoC will end up with
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a different C struct and format for the platform data.
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- The platform data is provided to drivers as a C structure. The driver
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must use the same structure to access the data. Since a driver
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normally also supports device tree it must use #ifdef to separate
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out this code, since the structures are only available in SPL.
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How it works
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------------
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The feature is enabled by CONFIG SPL_OF_PLATDATA. This is only available
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in SPL and should be tested with:
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#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
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A new tool called 'dtoc' converts a device tree file either into a set of
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struct declarations, one for each compatible node, or a set of
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U_BOOT_DEVICE() declarations along with the actual platform data for each
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device. As an example, consider this MMC node:
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sdmmc: dwmmc@ff0c0000 {
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compatible = "rockchip,rk3288-dw-mshc";
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clock-freq-min-max = <400000 150000000>;
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clocks = <&cru HCLK_SDMMC>, <&cru SCLK_SDMMC>,
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<&cru SCLK_SDMMC_DRV>, <&cru SCLK_SDMMC_SAMPLE>;
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clock-names = "biu", "ciu", "ciu_drv", "ciu_sample";
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fifo-depth = <0x100>;
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interrupts = <GIC_SPI 32 IRQ_TYPE_LEVEL_HIGH>;
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reg = <0xff0c0000 0x4000>;
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bus-width = <4>;
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cap-mmc-highspeed;
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cap-sd-highspeed;
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card-detect-delay = <200>;
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disable-wp;
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num-slots = <1>;
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pinctrl-names = "default";
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pinctrl-0 = <&sdmmc_clk>, <&sdmmc_cmd>, <&sdmmc_cd>, <&sdmmc_bus4>;
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vmmc-supply = <&vcc_sd>;
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status = "okay";
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u-boot,dm-pre-reloc;
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};
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Some of these properties are dropped by U-Boot under control of the
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CONFIG_OF_SPL_REMOVE_PROPS option. The rest are processed. This will produce
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the following C struct declaration:
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struct dtd_rockchip_rk3288_dw_mshc {
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fdt32_t bus_width;
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bool cap_mmc_highspeed;
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bool cap_sd_highspeed;
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fdt32_t card_detect_delay;
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fdt32_t clock_freq_min_max[2];
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struct phandle_2_cell clocks[4];
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bool disable_wp;
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fdt32_t fifo_depth;
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fdt32_t interrupts[3];
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fdt32_t num_slots;
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fdt32_t reg[2];
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bool u_boot_dm_pre_reloc;
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fdt32_t vmmc_supply;
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};
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and the following device declaration:
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static struct dtd_rockchip_rk3288_dw_mshc dtv_dwmmc_at_ff0c0000 = {
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.fifo_depth = 0x100,
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.cap_sd_highspeed = true,
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.interrupts = {0x0, 0x20, 0x4},
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.clock_freq_min_max = {0x61a80, 0x8f0d180},
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.vmmc_supply = 0xb,
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.num_slots = 0x1,
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.clocks = {{&dtv_clock_controller_at_ff760000, 456}, {&dtv_clock_controller_at_ff760000, 68}, {&dtv_clock_controller_at_ff760000, 114}, {&dtv_clock_controller_at_ff760000, 118}},
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.cap_mmc_highspeed = true,
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.disable_wp = true,
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.bus_width = 0x4,
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.u_boot_dm_pre_reloc = true,
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.reg = {0xff0c0000, 0x4000},
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.card_detect_delay = 0xc8,
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};
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U_BOOT_DEVICE(dwmmc_at_ff0c0000) = {
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.name = "rockchip_rk3288_dw_mshc",
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.platdata = &dtv_dwmmc_at_ff0c0000,
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};
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The device is then instantiated at run-time and the platform data can be
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accessed using:
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struct udevice *dev;
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struct dtd_rockchip_rk3288_dw_mshc *plat = dev_get_platdata(dev);
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This avoids the code overhead of converting the device tree data to
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platform data in the driver. The ofdata_to_platdata() method should
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therefore do nothing in such a driver.
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How to structure your driver
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----------------------------
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Drivers should always support device tree as an option. The of-platdata
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feature is intended as a add-on to existing drivers.
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Your driver should directly access the platdata struct in its probe()
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method. The existing device tree decoding logic should be kept in the
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ofdata_to_platdata() and wrapped with #ifdef.
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For example:
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#include <dt-structs.h>
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struct mmc_platdata {
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#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
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/* Put this first */
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struct dtd_mmc dtplat;
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#endif
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/*
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* Other fields can go here, to be filled in by decoding from
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* the device tree. They will point to random memory in the
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* of-plat case.
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*/
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int fifo_depth;
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};
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static int mmc_ofdata_to_platdata(struct udevice *dev)
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{
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#if !CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
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struct mmc_platdata *plat = dev_get_platdata(dev);
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const void *blob = gd->fdt_blob;
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int node = dev->of_offset;
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plat->fifo_depth = fdtdec_get_int(blob, node, "fifo-depth", 0);
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#endif
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return 0;
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}
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static int mmc_probe(struct udevice *dev)
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{
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struct mmc_platdata *plat = dev_get_platdata(dev);
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#if CONFIG_IS_ENABLED(SPL_OF_PLATDATA)
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struct dtd_mmc *dtplat = &plat->dtplat;
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/* Set up the device from the dtplat data */
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writel(dtplat->fifo_depth, ...)
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#else
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/* Set up the device from the plat data */
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writel(plat->fifo_depth, ...)
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#endif
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}
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static const struct udevice_id mmc_ids[] = {
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{ .compatible = "vendor,mmc" },
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{ }
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};
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U_BOOT_DRIVER(mmc_drv) = {
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.name = "mmc",
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.id = UCLASS_MMC,
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.of_match = mmc_ids,
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.ofdata_to_platdata = mmc_ofdata_to_platdata,
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.probe = mmc_probe,
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.priv_auto_alloc_size = sizeof(struct mmc_priv),
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.platdata_auto_alloc_size = sizeof(struct mmc_platdata),
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};
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In the case where SPL_OF_PLATDATA is enabled, platdata_auto_alloc_size is
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ignored, and the platform data points to the C structure data. In the case
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where device tree is used, the platform data is allocated, and starts
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zeroed. In this case the ofdata_to_platdata() method should set up the
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platform data.
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SPL must use either of-platdata or device tree. Drivers cannot use both.
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The device tree becomes in accessible when CONFIG_SPL_OF_PLATDATA is enabled,
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since the device-tree access code is not compiled in.
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Internals
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---------
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The dt-structs.h file includes the generated file
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(include/generated//dt-structs.h) if CONFIG_SPL_OF_PLATDATA is enabled.
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Otherwise (such as in U-Boot proper) these structs are not available. This
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prevents them being used inadvertently.
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The dt-platdata.c file contains the device declarations and is is built in
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spl/dt-platdata.c.
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Some phandles (thsoe that are recognised as such) are converted into
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points to platform data. This pointer can potentially be used to access the
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referenced device (by searching for the pointer value). This feature is not
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yet implemented, however.
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The beginnings of a libfdt Python module are provided. So far this only
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implements a subset of the features.
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The 'swig' tool is needed to build the libfdt Python module.
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Future work
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-----------
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- Add unit tests
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- Add a sandbox_spl functional test
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- Consider programmatically reading binding files instead of device tree
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contents
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- Drop the device tree data from the SPL image
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- Complete the phandle feature
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- Get this running on a Rockchip board
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- Move to using a full Python libfdt module
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--
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Simon Glass <sjg@chromium.org>
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6/6/16
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